The present invention relates to semiconductor devices and, more particularly, to a two-terminal device with a three-junction structure having the function of an overvoltage suppressor.
Overvoltage suppressors are circuit components that are especially employed when it is desired to avoid damage to a circuit that can be subjected to high overvoltages, either transient or continuous. Ideally, this circuit behaves like an open circuit under normal operating conditions, and like a short circuit when the voltage applied to its terminals exceeds a predetermined value (i.e.--a striking potential).
A three-junction device of known construction is shown in cross-section in FIG. 1. The device is formed on a P-type monocrystalline silicon substrate denoted by 1 on which an epitaxial layer 2 of silicon doped with N-type impurities has been allowed to grow and in which there has been formed, through diffusion operations with conventional processes known from planar technology, a circular region with a P-type conductivity indicated by 3 in the figure.
Within the region 3 of the structure depicted in FIG. 1 there is then formed, by diffusion with N-type impurities, a likewise circular region denoted by 4 in order to obtain a PNPN structure. A metallic layer 5, in ohmic contact with the surface portions of the regions 3 and 4, forms the emitter terminal of the device, and a metallic layer 6 in ohmic contact with the substrate 1 forms the collector terminal. The surface of the device, except for the zones of the metallic contacts, is covered with a silicon dioxide layer 7.
The operation of the device whose structure is shown in FIG. 1 can readily be understood by examining the equivalent electrical circuit shown in FIG. 3 where the emitter 10 of an NPN transistor TR2 corresponds to the region 4 of the structure; the base 11 of the transistor TR2 and the collector 15 of a PNP transistor TR1 correspond to the region 3 of the structure; the resistor R between the base 11 and the emitter 10 of the transistor TR2 represents the resistance distributed between the junction defined by the regions 3 and 4 and the emitter contact 5; the collector 12 of the transistor TR2 and the base 14 of the transistor TR1 correspond to the region 2 of the structure, and the emitter 13 of the transistor TR1 corresponds to the region 1 of the structure.
If one applies a voltage between the two electrodes 5 and 6 of the structure described above, that is--a positive voltage +V on the terminal 6, across the junction between the region 2 and the region 3 indicated by 8 an electrical field E will be established which causes the depletion of majority carriers in the region adjoining the junction (i.e.--a depletion layer) and which further causes the consequent creation of a space charge near the junction proper. If the voltage +V has a value such that the electrical field E exceeds a critical value determined by the physical and geometrical characteristics of the regions forming the junction, there occurs within the depletion region a chain reaction called an avalanche breakdown which is the cause of a sudden flow of current through the junction. To represent said effect operationally, a Zener diode D.sub.z is shown in FIG. 3 between the base 11 of the transistor TR2 (region 3 of the structure) and the collector 12 of the transistor TR2, (region 2 of the structure).
Under these conditions, current flows between the two electrodes 5 and 6 of the device, because both transistors TR1 and TR2 are conducting. It is known that the avalanche breakdown in a planar junction protected on the surface by an insulating layer of silicon dioxide, such as the layer 7 depicted in FIG. 1, occurs on the surface due to the presence of disturbing carriers in the insulating layer. They have the effect of limiting the depletion layer to the surface thereof and thus have the effect of reducing the breakdown voltage, or the breakdown of the junction. It is also known that the repetition of such a phenomenon tends to alter the characteristics of the reverse-biased junction and to modify its breakdown voltage.
To make the above-mentioned phenomenon less sensitive, in the structure of FIG. 1, the metallic layer 5 in ohmic contact with the surface portions of the regions 3 and 4 can be extended to above the insulating layer 7 until it rises above part of the region 2. In practice, this metallic layer so extended, which used to be called a field plate when the voltage applied between the electrodes 5 and 6 has the polarity indicated in the electrical circuit of FIG. 3, serves to remove possible accumulations of carriers from the surface of the oxide 7 and allows the surface of the oxide 7 and the underlying doped silicon surface to be at an equipotential level, so that the breakdown no longer occurs on the surface, but inside the junction 8 and at a higher voltage. Specifically, unless phenomena of a different nature occur, this takes place on the surface of maximum curvature of the junction and at a voltage which remains stable throughout the life of the device. However, such a voltage cannot be accurately defined in the design phase due to the changeability of the fabrication parameters.
It is also known that in order to bring about the conduction between the regions 1 and 3 of a structure of the type illustrated in FIG. 1, as is necessary for the firing of the conduction between the two electrodes 5 and 6 of the device, one can act upon the fabrication parameters, particularly on the resistivity and on the thickness of the epitaxial layer 2, so that the conduction through the junction 8 due to an avalanche breakdown cannot be initiated, because it is anticipated by another phenomenon which modifies the operating conditions of the structure. Said phenomenon, known as punch-through, occurs when the depletion zone of the junction formed between the regions 1 and 2 extends throughout the epitaxial layer delimited by the region 3 in order to cause a short circuit between the regions 1 and 3. However, even in this case, it is not possible to accurately define the striking potential in the design phase due to the changeability of the fabrication parameters.